Numerous systems exist wherein a rotating or spinning body is mounted for rotation on a fixed body to which the spinning body is connected by a shaft that is coincident with a spin axis of the rotating body. If the spinning body is inertially asymmetrical with respect to its spin axis, i.e., it spins about an axis that is not one of its principal moment of inertia axes, bearing forces are exerted by the fixed body on the spinning body in a plane containing the spin axis of the spinning body. Equal and opposite forces are exerted on the fixed body by the spinning body. These forces are generally classified as dynamic unbalance forces. Any body which rotates about a fixed axis and is connected to a fixed body exerts dynamic unbalance forces on the fixed body because manufacturing imperfections make it impossible to produce a truly symmetrical body and to locate a spin axis of the rotating body exactly at one of its axes of principal moments of inertia. Asymmetry also arises in spinning bodies which appear to be symmetrical if the symmetrical body is mounted so that the axis of symmetry thereof is not aligned with the spin axis. For example, if a spinning parabolic antenna reflector is mounted on a shaft carried by a fine platform of a spacecraft, the antenna reflector is considered to be asymmetrical with respect to a shaft which connects it to the fixed body unless the antenna bore sight axis is aligned with the shaft.
One accepted prior art method of reducing the dynamic unbalance forces applied by a spinning or rotating body to a fixed body on which the rotating body is mounted involves spinning the rotating body on a shaft connected to a fixed platform of a machine designed specifically for measuring dynamic unbalance. As the spinning body rotates, the unbalance exerted by it on the fixed platform is measured. As a result of the measurement, small weights are empirically added and subtracted to reduce the unbalance, until the unbalance is within an acceptable region.
Several disadvantages are associated with this prior art technique. In particular, spinning bodies which must be compensated for dynamic unbalance can be relatively large and therefore cannot be mounted conveniently, if at all, on presently available machines designed to measure dynamic unbalance. This is particularly true of spinning antennae mounted on spacecraft, since such antennae now frequently have reflectors with diameters of approximately three meters. It has also been found that the testing machines have limited sensitivity, i.e., they are capable of achieving only certain levels of unbalance measurements. These levels may not be acceptable in connection with parabolic reflectors employed on spacecraft. For space applications involving large structures, such as parabolic reflectors, certain deformations which occur in the structure when they are tested on earth do not occur when the structures are in space on a spacecraft.
It is important in spacecraft to minimize as much as possible, and attempt to eliminate completely, dynamic unbalance forces imparted by a spinning body to a fixed body on which the spinning body is mounted. This is because the fixed body usually carries a variety of instruments used for scientific and other applications, such as meteorology. Instruments having spinning components, such as antenna reflectors, are mounted on the same fixed bodies as other, highly sensitive instruments which must be very stably mounted. As the spinning instruments or bodies become larger, it becomes more difficult to mount them on the same platform as the sensitive instruments because a slight dynamic unbalance force imparted by the spinning body or instrument on the fixed body causes the fixed body or platform to wobble or jitter. The wobble or jitter of the fixed body disturbs the sensitive instruments and is likely to affect measurements thereof adversely.
In one particular spacecraft, a parabolic antenna reflector having a diameter of two to four meters spins at sixty revolutions per minute about a shaft mounted on a fixed platform. The reflector has a bore sight axis that is offset forty-five degrees from the spin axis, to provide a spinning structure that is asymmetrical and exerts relatively large dynamic unbalance forces on the fixed body. The fixed body also carries instruments that are highly sensitive and which should not rotate more than three to fifteen arc seconds if they are to work properly. Thus, compensation must be provided for the dynamic unbalance exerted by the antenna reflector on the fixed body.
While certain systems have been designed to provide in place compensation for the dynamic unbalance, the prior art systems are not believed sufficiently practical to be effective. One prior art system utilizes two orthogonal, passive spring-mass vibrator systems located on the fixed body. The vibrators are lossless, mechanical resonators, which resonate at the spin frequency of the rotating body.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for compensating for dynamic unbalance forces exerted by a spinning body on a fixed body that carries the spinning body.
Another object of the present invention is to provide a new and improved apparatus for and method of compensating for dynamic unbalance forces exerted by a relatively large spinning body on a fixed body carrying the spinning body so that sensitive instruments on the fixed body are not disturbed by the dynamic unbalance.